Persistent vegetative states lasting longer than 12 months have long been considered irreversible. But a 35-year-old man severely injured in a car accident was partially revived by vagus nerve stimulation after lying in a vegetative state for 15 years.

The technique has been in use for many years for treating people with epilepsy or depression. But this is the first time that doctors attempted to treat a vegetative patient with the technique.

The vagus nerve connects the human brain stem to the heart, lungs, and digestive tract. It's the longest nerve in the body's autonomous nervous system, which mostly regulates unconscious functions like heart rate, digestion, and breathing.

Angela Sirigu, who led the research at the Institute of Cognitive Sciences – Marc Jeannerod in Lyon, France, said the technique could trigger a radical change in neurological treatments worldwide.

“Brain plasticity and brain repair are still possible even when hope seems to have vanished,” Sirigu said in a statement accompanying publication of research describing the procedure.

The research team began the experiment by looking for a particularly difficult case, to reduce the possibility that any improvements weren't simply a matter of chance and good timing. The patient chosen for the experiment had shown no signs of improvement in 15 years.

Doctors then implanted a vagus nerve simulator in the man's chest designed to send small pulses of electricity up the vagus nerve and into the brain.

After a month of constant stimulation, the patient's movements and brain activity improved significantly. He responded to simple commands, such as following an object with his eyes and turning his head upon request.

Computer monitoring of the patient’s brain activity confirmed major changes took place. Imaging scans showed increased metabolic activity in areas of the brain associated with movement, awareness, and sensation. A series of electroencephalogram tests suggested that the patient had improved from a “vegetative state” to a “minimally conscious state.”

By stimulating the vagus nerve, “it is possible to improve a patient's presence in the world,” Sirigu said.

An estimated 25,000 people in the US lie in a vegetative state at any given time.

While the new study marks a positive development, researchers caution that the study is, by design, extremely limited in scope.

“We need to be a little cautious about this, because it's just one patient,” said neurologist Hae Won Shin, an associate professor at the University of North Carolina School of Medicine who was not involved in the research. “I'm really glad to hear that the patient responded positively to vagus nerve stimulation treatment after 15 years in a vegetative state, but it's only one case.”

The researchers are currently planning a larger collaborative study to confirm the therapeutic potential of VNS for patients in a vegetative state. The initial study was supported by France's National Center for Scientific Research, the French National Research Agency, and by a grant from the University of Lyon

Hae, who specializes in epileptic disorders, said vagus nerve stimulation has a track record of proven efficacy in treating certain disorders — but there's a caveat: No one is quite sure how it works.

“VNS has been around a long time for people who have epilepsy and also depression,” she said. “We've had all of these patients [respond to treatment], but we still don't really know why or how it is working. There are a lot of different theories. Something is happening, but we don't know how it is happening.”

Still, Hae said, if the French research team can replicate their outcomes in a larger study, the implications would be profound.

“If we find the treatment works on other patients,” she said, “it would truly be a breakthrough.”